The aim of the present study was to examine whether parameters of isolated mitochondria could account for the in vivo maximum oxygen uptake (O_2 max) of human skeletal muscle. O_2 max and work performance of the quadriceps muscle of six volunteers were measured in the knee extensor model (range 10-18 mmol O₂ · min¹ · kg¹ at work rates of 22-32 W/kg). Mitochondria were isolated from the same muscle at rest. Strong correlations were obtained between O_2 max and a number of mitochondrial parameters (mitochondrial protein, cytochrome aa₃, citrate synthase, and respiratory activities). The activities of citrate synthase, succinate dehydrogenase, and pyruvate dehydrogenase, measured in isolated mitochondria, corresponded to, respectively, 15, 3, and 1.1 times the rates calculated from O_2 max. The respiratory chain activity also appeared sufficient. Fully coupled in vitro respiration, which is limited by the rate of ATP synthesis, could account for, at most, 60% of the O_2 max. This might be due to systematic errors or to loose coupling of the mitochondrial respiration under intense exercise.

skeletal muscles; maximal oxygen uptake; work rate; respiration; adenosine triphosphate synthesis